Bug 1884596 - Toggle dom.disable_window_flip to true. r=mkmelin

m-c finally wants to drop the `dom.disable_window_flip` preference in
Bug 1773079 and it will be permanently set to `true`.

It is currently set to `false` for us, but it does not look like
flipping it to `true` causes any issues.

Differential Revision: https://phabricator.services.mozilla.com/D204176

--HG--
extra : amend_source : 6a5d5485be3713665634857a2e74f958ff9c5be7

mail/app/profile/all-thunderbird.js

@@ -1406,3 +1406,7 @@ pref("mailnews.imap.jsmodule", false);
 // 2: icons only
 // 3: text only
 pref("toolbar.unifiedtoolbar.buttonstyle", 0);
+
+// Bug 1773079 : check if true causes issues for Thunderbird
+// prevent JS from monkeying with window focus, etc
+pref("dom.disable_window_flip", true);